Electrostatic Analyzers:
Electrostatic ion mirrors may be employed in electrostatic ion traps (E-traps), open electrostatic traps (Open E-traps), and multi-reflecting time-of-flight mass spectrometers (MR-TOF MS). In all three cases, pulsed ion packets experience multiple isochronous reflections between parallel grid-free electrostatic ion mirrors spaced by a field-free region.
MR-TOF:
In MR-TOF, ion packets propagate through the electrostatic analyzer along a fixed flight path from an ion source to a detector, and ions' m/z ratios are calculated from flight times. SU1725289, incorporated herein by reference, introduces a scheme of a folded path MR-TOF MS, using two-dimensional gridless and planar ion mirrors. Ions experience multiple reflections between planar mirrors, while slowly drifting towards the detector in a so-called shift direction. The number of reflections is limited to avoid spatial spreading of ion packets and their overlapping between adjacent reflections. GB2403063 and U.S. Pat. No. 5,017,780, incorporated herein by reference, disclose a set of periodic lenses within planar two-dimensional MR-TOF to confine ion packets along the main zigzag trajectory. The scheme provides fixed ion path and allows using many tens of ion reflections.
In co-pending applications P129429 (E-trap; U.S. patent application Ser. No. 13/522,458, now U.S. Pat. No. 9,082,604), P129992 (open E-trap; U.S. patent application Ser. No. 13/582,535, now published as U.S. Publication No. 2013/0056627), P130653 (MR-TOF; U.S. patent application Ser. No. 13/695,388, now U.S. Pat. No. 8,853,623) and provisional application 61/541,710 (Cylindrical analyzer; now filed as U.S. patent application Ser. No. 14/441,700 and published as WO 2014/074822), incorporated herein by reference, there is disclosed a hollow cylindrical analyzer formed by two sets of coaxial rings having a cylindrical field volume. The analyzer provides an effective folding of ion trajectory per compact analyzer size.
E-Traps:
In E-traps, ions may be trapped indefinitely. An image current detector is employed to sense the frequency of ion oscillations as suggested in U.S. Pat. No. 6,013,913A, U.S. Pat. No. 5,880,466, and U.S. Pat. No. 6,744,042, incorporated herein by reference. Such systems are referred to as Fourier Transform S-traps. To improve the space charge capacity of E-traps, the co-pending application P129429 (now U.S. Pat. No. 9,082,604), incorporated herein by reference, describes extended E-traps employing two-dimensional fields of planar and hollow cylindrical symmetries.
E-Trap MS with a TOF detector resemble features of both MR-TOF and E-traps. Ions are pulse-injected into a trapping electrostatic field and experience repetitive oscillations along the same ion path, so the technique is called I-path E-trap. Ion packets are pulse ejected onto the TOF detector after some delay corresponding to a large number of cycles. In FIG. 5 of GB2080021 and in U.S. Pat. No. 5,017,780, incorporated herein by reference, ion packets are reflected between coaxial gridless mirrors.
The co-pending application P129992 (now published as U.S. Publication No. 2013/0056627), incorporated herein by reference, describes an open E-trap, where ions propagate through an analyzer, but the flight path is not fixed—it may contain an integer number of oscillations within some span before ions reach a detector.
Gridless Ion Mirrors:
To increase resolution of TOF MS, U.S. Pat. No. 4,072,862, incorporated herein by reference, discloses a grid covered dual stage ion mirror which provides second order time per energy focusing. Multiple reflections may be arranged within grid-free ion mirrors to prevent ion losses. U.S. Pat. No. 4,731,532, incorporated herein by reference, discloses ion mirrors with purely retarding fields in which a stronger field is located at the mirror entrance to facilitate spatial ion focusing. As disclosed, the mirrors are capable of reaching either a second order time per energy focusing T|KK=0 or a second order time-spatial focusing T|YY=0, but such are unable to reach both conditions simultaneously. SU1725289, incorporated herein by reference, employs similar ion mirrors. In addition, DE10116536, incorporated herein by reference, proposed gridless ion mirrors with an attracting potential at the mirror entrance which improved time per energy focusing. Paper by Pomozov et al JTP (Russian), 2012, V. 82, #4, incorporated herein by reference, demonstrates reaching third order energy focusing in such mirrors in coaxial symmetry. Paper by M. Yavor et al., Physics Procedia, v.1 N1, (2008) 391-400, incorporated herein by reference, provides details of geometry and potentials for planar mirrors and demonstrates reaching simultaneously: spatial focusing; third order time per energy focusing; and second-order time-spatial focusing with compensation of second order cross-terms. However, to sustain resolving power above 100,000 the energy tolerance is limited to about 7%. This limits the maximal strength of electric field in pulsed ion sources and thus the ability of compensating so-called turn around time. As a result, the flight path and flight time in MR-TOF analyzers have to be longer, which in turn limits duty cycle of MR-TOF.
Thus, the prior ion mirrors reach third order time per energy focusing only. Therefore, there is a need for improving aberration coefficients, isochronicity and energy tolerance of ion mirrors.